MS51021 Set Screw Decoder: Dash Number & Size Lookup

What Size Is MS51021-56?

MS51021-56 is a 1/4-20 × 1/4 in stainless cup-point socket set screw. MS51021 dash numbers carry no code: each one is a lookup into Table I of the standard. Numbers 1–96 are plain screws. Numbers 101–142 have a self-locking element. Type a part number and the decoder gives the thread, length, socket size, cup point limits, material and status. NASM51021 numbers work too: they are the same.

Going the other way? Type a size, such as 1/4-20 x 1/2 or 8-32 x 3/16, to get the part number.

MS51021 DECODER

CRES cup-point set screws · UNC-3A MS51021 Rev C, Dec 1972

Part number or size

Examples:

Part number	Thread	Length	L in	Type	Socket J	Cup Ø C

How an MS51021 Part Number Works

1–96 plain (no locking element) 101–142 self-locking, 4-40, 6-32 and 8-32 only

There is no letter for self-locking. A plain screw and the self-locking screw of the same size have unrelated dash numbers: MS51021-32 and MS51021-133 are both 8-32 × 1/4 in.

Dash Numbers (Table I): Plain / Self-Locking

Length L, in (tol)	2-56	4-40	6-32	8-32	10-24	1/4-20	5/16-18	3/8-16	1/2-13
.125 (1/8) ±.010	1	9 / 101*	21 / 111*	131*
.188 (3/16) ±.010	2	10 / 102	22 / 112*	31 / 132*	42	55
.250 (1/4) ±.010	3	11 / 103	23 / 113	32 / 133	43	56	68
.312 (5/16) ±.010		12 / 104	24 / 114	33 / 134	44	57	69	80
.375 (3/8) ±.010		13 / 105	25 / 115	34 / 135	45	58	70	81	91
.438 (7/16) ±.010		14 / 106	116	136
.500 (1/2) ±.010		15 / 107	117	36 / 137	47	60	72	83	93
.562 (9/16) ±.010		108	118	138
.625 (5/8) ±.010		109	119	139	48	61	73	84	94
.750 (3/4) ±.020			120	140	49	62	74	85	95
.875 (7/8) ±.020			121	141
1.000 (1) ±.020				142		63	75	86	96

Columns are the thread size (all UNC-3A). Upright numbers are plain; italic numbers are self-locking. * Above the dashed line in Table I: MIL-F-18240 sets no torque requirement at these lengths, only a positive indication of torque (Note 8e). Blank cells are not made.

Socket and Cup Point Dimensions (Table I), inches

Nominal size	Threads per inch	Socket J, nom (key)	Cup Ø C	Cup Ø C₁	Cup Ø C₂	Point length S
.086 (2-56)	56	0.035 (.035)	0.039–0.047	0.038–0.043	0.038–0.043	0.007–0.010
.112 (4-40)	40	0.050 (.050)	0.051–0.061	0.051–0.056	0.054–0.059	0.008–0.013
.138 (6-32)	32	0.062 (1/16)	0.064–0.074	0.062–0.069	0.068–0.074	0.012–0.017
.164 (8-32)	32	0.078 (5/64)	0.076–0.087	0.074–0.082	0.084–0.090	0.016–0.021
.190 (10-24)	24	0.094 (3/32)	0.088–0.102	0.086–0.095	0.095–0.101	0.019–0.024
.250 (1/4-20)	20	0.125 (1/8)	0.118–0.132	0.114–0.125	0.150–0.156	0.022–0.027
.3125 (5/16-18)	18	0.156 (5/32)	0.156–0.172	0.144–0.156	0.185–0.190	0.033–0.038
.375 (3/8-16)	16	0.188 (3/16)	0.194–0.212	0.174–0.187	0.236–0.241	0.036–0.041
.500 (1/2-13)	13	0.250 (1/4)	0.270–0.291	0.235–0.250	0.328–0.333	0.049–0.054

Limits are min–max. Multiply by 25.4 for mm (1/4-20 socket: .125 in = 3.18 mm). C is for the plain, knurled and W-point cups; C₁ for the small, hexagon and internal knurled cups; C₂ and S for the Nu-cup.

MS18064 Replacements (Table II)

Cancelled	Use instead	Size
MS18064-1	MS51021-101	4-40 × 1/8
MS18064-5	MS51021-102	4-40 × 3/16
MS18064-10	MS51021-103	4-40 × 1/4
MS18064-16	MS51021-104	4-40 × 5/16
MS18064-22	MS51021-105	4-40 × 3/8
MS18064-89	MS51021-106	4-40 × 7/16
MS18064-33	MS51021-107	4-40 × 1/2
MS18064-41	MS51021-108	4-40 × 9/16
MS18064-46	MS51021-109	4-40 × 5/8
MS18064-2	MS51021-111	6-32 × 1/8
MS18064-6	MS51021-112	6-32 × 3/16
MS18064-11	MS51021-113	6-32 × 1/4
MS18064-17	MS51021-114	6-32 × 5/16
MS18064-23	MS51021-115	6-32 × 3/8
MS18064-90	MS51021-116	6-32 × 7/16
MS18064-34	MS51021-117	6-32 × 1/2
MS18064-42	MS51021-118	6-32 × 9/16
MS18064-47	MS51021-119	6-32 × 5/8
MS18064-54	MS51021-120	6-32 × 3/4
MS18064-61	MS51021-121	6-32 × 7/8
MS18064-3	MS51021-131	8-32 × 1/8
MS18064-7	MS51021-132	8-32 × 3/16
MS18064-12	MS51021-133	8-32 × 1/4
MS18064-18	MS51021-134	8-32 × 5/16
MS18064-24	MS51021-135	8-32 × 3/8
MS18064-30	MS51021-136	8-32 × 7/16
MS18064-35	MS51021-137	8-32 × 1/2
MS18064-43	MS51021-138	8-32 × 9/16
MS18064-48	MS51021-139	8-32 × 5/8
MS18064-55	MS51021-140	8-32 × 3/4
MS18064-62	MS51021-141	8-32 × 7/8
MS18064-67	MS51021-142	8-32 × 1

MS18064 was cancelled in part. Only the numbers above moved to MS51021, all as self-locking screws.

Reference Guide: MS51021 Set Screws

Reading the Number

The part number is the MS number plus the dash number (Note 7). Nothing is coded in the dash number. Its place in Table I sets the thread size, the length and whether the screw is self-locking.

MS51021-1: 2-56 × 1/8 in, plain (the sheet's own example).
MS51021-56: 1/4-20 × 1/4 in, plain.
MS51021-134: 8-32 × 5/16 in, self-locking.

Gaps in the numbering

Many numbers are not in Table I: 4–8, 16–20, 26–30, 35, 37–41, 46, 50–54, 59, 64–67, 71, 76–79, 82, 87–90, 92, 97–100, 110 and 122–130. For example, there is no 8-32 × 7/16 in plain screw (35), and 2-56 stops at 1/4 in. The decoder says so and lists the nearest numbers.

MS51022 screws were cancelled after 29 January 1964. They are replaced by MS51021 screws with the same dash numbers.

Material, Finish and Threads

Material: corrosion-resisting (chrome-nickel) steel, 300 series, per the procurement specification, FF-S-200 (Note 1).
Finish: passivated per QQ-P-35 (Note 2).
Threads: UNC-3A per MIL-S-7742 (Note 3).
Magnetic permeability: 2.0 max (air = 1.0), checked with an indicator per MIL-I-17214 (Note 4).

Cup point: 118° included angle. The drawing shows seven optional cup types: plain, small, knurled, hexagon, W point (90°), Nu-cup (60°) and internal knurled. The sheet does not say who chooses the type.
Chamfers: 35° +10°/−5° at the socket end, 45° +5°/−0° at the point end. Other angles ±5° (Note 6).
Length tolerance: ±.010 in up to 5/8 in long, ±.020 in for 3/4 to 1 in.

Self-Locking Element

Self-locking screws (101–109 for 4-40, 111–121 for 6-32, 131–142 for 8-32) carry a locking element per MIL-F-18240 (Note 8a). Design and usage limits, and the location of the locking area, follow MS15981 (Notes 8b and 8c).

Ring gage: the locking region must pass a ring gage of the nominal size plus .010 in, freely or with finger pressure (Note 8d).
Short screws: MIL-F-18240 sets no torque requirement for lengths above the dashed line in Table I (101, 111, 112, 131, 132). Only a positive indication of torque is required (Note 8e).

Status: MS51021 and NASM51021

MS51021 was first approved 7 May 1958 and revised in 1964 (A), 1967 (B) and 29 December 1972 (C). Notice 1 cancelled it on 30 September 1999 and superseded it by NASM51021. The notice states that the part numbers are identical, so MS51021-1 and NASM51021-1 are the same screw.

Source: MS51021C sheets 1 and 2, and Notice 1, from the DLA ASSIST database (Distribution A). It supersedes MS51022 and, in part, MS18064, BCTX1.2 and BCUX5. Federal supply class 5305.

MS51021 / NASM51021 Set Screw Part Number Decoder: Thread, Length, Socket, Cup Point and Self-Locking

Reading the Number

Material, Finish and Threads

Self-Locking Element

Status: MS51021 and NASM51021

Cite This Work